Method and device for the verification and identification of a measuring device

ABSTRACT

The invention concerns a method and a reference object for the verification and identification of a measuring device for measuring the spatial position of one or several points situated in the measuring volume ( 9 ) of the measuring device, whereby the measuring device is calibrated, according to which calibration a mathematical model is calculated which makes it possible to determine the spatial position of a point perceived by the measuring device, whereby a reference object ( 1 ) with predetermined dimensions is provided in the measuring volume ( 9 ), and, next, the position of several points of this reference object ( 1 ) is determined by means of the measuring device, whereby the mutual situation of the thus determined position of the points of the reference object ( 1 ) is calculated and compared to the actual dimensions of the reference object ( 1 ) and, on the basis of the thus established deviations between the measured mutual position of the points of the reference object ( 1 ) and the actual dimensions of the reference object ( 1 ), said model is adjusted.

The invention concerns a method for the verification and identificationof a measuring device for measuring the spatial position of one orseveral points situated in the measuring volume of the measuring device,whereby the measuring device is calibrated, according to whichcalibration a mathematical model is calculated which makes it possibleto determine the spatial position of a point perceived by the measuringdevice.

The present measuring devices have to be calibrated again each time themeasuring accuracy of the device is insufficient, for example as aresult of thermal or mechanical shocks occurring during the conveyanceof the measuring device. Such a calibration is very time-consuming andmay take several days.

For optical measuring systems, which determine the position of a pointby means of one or several cameras, a large number of points aremeasured during the calibration of the measuring device, situated in agrid with known dimensions. By means of the thus measured points isgenerated a mathematical model which makes it possible to calculate thethree-dimensional co-ordinates of a point perceived by the cameras.

When the mutual positions of the cameras of the measuring device altersomewhat, for example due to thermal or mechanical loads during theirconveyance, a measuring error in the order of magnitude of 0.5 mm mayoccur. According to the present state of the art, the measuring devicemust then be entirely calibrated again.

With conventional co-ordinate measuring machines (“CMM”), the points ofan object whose dimensions or position needs to be determined, aretouched by means of a measuring feeler mounted on a robot arm, inparticular on what is called a manipulator. Should there be a collisionbetween the object to be measured and the manipulator or the measuringfeeler, a new calibration will be required. During such a calibration,one or several reference bars having a certified length are measured inturns according to the three orthogonal axes of the co-ordinate systemused by the co-ordinate measuring machine, and subsequently theparameters of the co-ordinate measuring machine will be adjusted on thebasis of the measurements which have been carried out on the referencebar. This is also a very time-consuming procedure.

The invention aims to remedy these disadvantages by proposing a methodwhich makes it possible to improve the measuring accuracy of a measuringdevice in a very simple manner when the latter has been exposed tothermal or mechanical shocks or to other phenomenons of de-calibration,without any need for re-calibration arising.

To this aim, a reference object with predetermined dimensions isprovided in the measuring volume, and then the position of severalpoints of this reference object is determined by means of the measuringdevice, whereby the mutual situation of the thus determined position ofthe points of the reference object is calculated and compared to theactual dimensions of the reference object and, on the basis of the thusestablished deviations between the measured mutual position of thepoints of the reference object and the actual dimensions of thereference object, said model is adjusted.

Practically, the position of said points of the reference object isdetermined for different orientations of the reference object.

According to a preferred embodiment, reference points are provided onsaid reference object, and the position of these reference points ismeasured by means of the above-mentioned measuring device, such that theorientation and/or the position of the measuring object will bedetermined when the position of said points thereof is determined.

The invention also concerns a reference object for the application ofthe method according to the invention.

This reference object is characterised in that it comprises at least tworeference elements whose mutual position is determined, whereby thesereference elements are connected to one another via a connecting piecehaving a negligible thermal coefficient of expansion.

According to an advantageous embodiment of the reference objectaccording to the invention, it will have reference points which make itpossible to determine its movement. These reference points consist forexample of light-emitting diodes (LED's).

Other particularities and advantages of the invention will become clearfrom the following description of a few specific embodiments of themethod and the reference object according to the invention; thisdescription is given as an example only and does not limit the scope ofthe claimed protection in any way; the reference figures used hereafterrefer to the accompanying drawings.

FIG. 1 is a schematic side view of a reference object according to afirst embodiment of the invention.

FIG. 2 is a schematic representation of a measuring device with areference object according to the invention.

FIG. 3 is a schematic side view of a reference object according to asecond embodiment of the invention.

FIG. 4 is a schematic side view of a reference object according to athird embodiment of the invention.

In the different drawings, the same reference figures refer to identicalor analogous elements.

FIG. 1 represents a first embodiment of the reference object accordingto the invention. This reference object 1 comprises two sphericalreference elements 2 and 3 which are mutually connected by means of aconnecting piece which is made of a longitudinal, round bar 4. This bar4 is preferably made of carbon fibres and fibres made ofpolyparaphenylene terephthalamide, extending in the longitudinaldirection of the bar 4 or which are wound in said bar 4 according to acylindrical helical line. Said fibres made of polyparaphenyleneterephthalamide are known under the brand name Kevlar®. The bar 4 maypossibly consist of composition materials on the basis of these fibres.

The dimensions of the spherical reference elements 2 and 3 and theirmutual distance and position are determined with great accuracy andpossibly certified by an authorised institution.

In order to make sure that the length of the bar 4 is nottemperature-dependent in any way, it is especially composed of fibreshaving a negative thermal expansion coefficient, such as for examplefibres made of polyparaphenylene terephthalamide, and fibres having apositive thermal expansion coefficient, such as for example carbonfibres. The amount of fibres with a positive expansion coefficient andthe amount of fibres with a negative expansion coefficient is selectedsuch that the total expansion coefficient of the bar is practicallyequal to zero or at least negligible.

Thus, the percentage of the respective fibres is selected such that itis inversely proportionate to the respective expansion coefficientthereof.

In order to avoid any possible influences of the atmospheric humidity onthe qualities of the bar 4, the latter is preferably covered with awaterproof coating.

Each of the spherical reference elements 2 and 3 consists of a sphericalball made for example of ceramics, steel or artificial ruby.

According to the method of the invention, such a reference object isused for the verification and the identification of a measuring device.During the verification will be checked whether the measuring deviceallows for a correct measuring and makes it possible to determine theposition with the required accuracy. Said accuracy is for example in theorder of magnitude of 0.05 mm for a measuring volume of 10 m³.

During the identification, the model of the measuring device which hasbeen determined during the calibration thereof is adjusted somewhat onthe basis of the result of said verification in order to obtain therequired measuring accuracy. This is done for example by calculating thevalue of parameters of said model and their influence on the recordedmeasuring, and by subsequently adjusting these parameters in the model.In an optical measuring system, such parameters may for example be themutual position of and the angles between the different cameras, or thelens qualities of the cameras, etc.

FIG. 2 schematically represents a measuring device by way of example.This measuring device consists of an optical measuring system with threecameras 5, 6 and 7, co-operating with the arithmetic unit of a computer8. The space situated within the field of vision of the cameras 5, 6 and7, in which the position of the points can be measured by means of themeasuring device, forms what is called a measuring volume 9.

When measuring the spatial position of the points situated in themeasuring volume 9, use is made of a holder 11 upon which are providedseveral reference points 12, 13 and 14, each formed of a light-emittingdiode. The holder 11 comprises a measuring feeler 20 with which it isplaced against a point of an object to be measured. Next, the positionof the reference points 12, 13 and 14 of the holder 11 is measured, inorder to determine the position and orientation of the latter, andconsequently find out the position of the measured point. Such a holder11 is known as such and is described for example in patent document WO98/48241.

As explained in the introduction of the description, such a measuringdevice is calibrated by measuring the position of a large number ofpoints situated in an orthogonal grid whose dimensions are known. Bymeans of the thus measured points is generated a mathematical modelwhich makes it possible to determine the three-dimensional co-ordinatesof any point whatsoever situated in the above-mentioned measuring volume9.

After the measuring device has been conveyed, or if it has been subjectto thermal shocks, a verification and an identification of the measuringsystem will be required. A reference object 1 is hereby placed in themeasuring volume 9, and the known dimensions thereof are measured bymeans of the measuring system. The reference object 1, which isrepresented in FIG. 2, has already been described above.

This reference object 1 is mounted on a standard 10, such that it cantake a fixed position in said measuring volume 9. Next, the positions ofa number of points on the surface of the spherical reference elements 2and 3 are measured by means of the above-mentioned holder 11.

Preferably, the position of at least four points of each referenceelement 2 and 3 is measured, such that, by means of the above-mentionedcomputer, the exact surface and possibly the middle point thereof can becalculated, as well as the distance between these reference elements 2and 3 and their respective diameters.

The dimensions of the reference object 1 which are thus determined bymeans of the measuring device are then compared to its exact dimensions.In particular, the distance between the reference elements 2 and 3measured by the measuring device is compared to the actual distancebetween them which was determined beforehand.

This measuring of the reference object 1 and the comparison to itsactual dimensions is preferably carried out for different orientationsor positions of the reference object 1 in the above-mentioned measuringvolume 9. Thus, the reference object 1 must not extend according to theorthogonal axes used by the measuring device, and the reference object 1can for example be measured according to some arbitrary orientations.

On the basis of the thus established deviations of the measuring resultsin relation to the actual dimensions of the reference object 1, theabove-mentioned mathematical model is finally adjusted somewhat, forexample by applying a correction factor for certain parameters thereof,so as to improve the accuracy of the measuring device, such that it isjust as good as the measuring accuracy obtained after a calibration ofthe measuring device. When for example one of the cameras of themeasuring system has moved over a certain angle in relation to the othercameras due to a mechanical shock, the influence thereof is rectified byrectifying the above-mentioned model, on the basis of the establisheddeviations, without any entirely new calibration of the device beingrequired.

The method above can possibly be repeated several times, such that aniterative process is obtained for the adjustment of said mathematicalmodel.

In some cases, the only thing that is required is the identification ofcertain parameters of the measuring device, namely of those parameterswhich have for example the greatest influence on the accuracy of theposition measurements, such as the mutual angles of the cameras in anoptical measuring device. In such a case, only these parameters will beadjusted in said mathematical model at the time of the identification.

By making use of the above-mentioned holder 11 to determine thedimensions and the mutual position of the reference elements 2 and 3 ofthe reference object 1, measuring errors occurring due to the use ofthis holder 11 during the measurement of the position of the points inthe above-mentioned measuring volume 9, and which have possibly not beentaken into account during the calibration of the measuring device, arerectified as well.

The above-mentioned reference object can be made in all sorts of shapesand dimensions. FIG. 3 represents a second embodiment of the referenceobject 1 according to the invention. This reference object 1 differsfrom the preceding embodiment in that each of said reference elements 2and 3 are formed of a plate in which is provided a recess 15, 16respectively, for example in the shape of a cylindrical bore hole. Thedimensions of these recesses 15 and 16 are selected such that ameasuring feeler, provided on the above-mentioned holder 11, can beprovided in these recesses 15 and 16 such that it practically fits.

Thus, with the method according to the invention, the distance betweenthese two recesses 15 and 16 is measured by means of said holder 11 bysuccessively placing the measuring feeler in each of these recesses andthus determining their position.

On the basis of the deviations found between the measured distancebetween said recesses 15 and 16 and the actual value of this distance,the above-mentioned mathematical model is adjusted.

FIG. 4 represents a third embodiment of the reference object 1 accordingto the invention. This reference object 1 corresponds to the referenceobject represented in FIG. 1, but it is further provided with referencepoints 17, 18 and 19 which can be observed by the above-mentionedcameras. Each of these reference points 17, 18 and 19 is preferablyformed of a light-emitting diode (LED).

The use of such a reference object 1 offers the advantage that, when themethod according to the invention is applied, it is not necessary tohold this reference object in a fixed position in the measuring volume 9when the distance between the reference elements 2 and 3 thereof isdetermined by means of the measuring device.

The reference object 1 is thus mounted in said measuring volume 9, forexample in a non-fixed manner, or it is held by a person in thismeasuring volume 9, while the position of the reference elements 2 and 3is being determined by means of the measuring device. The position ofthe reference points 17, 18 and 19 of the reference object 1 is herebyobserved as well by said cameras of the measuring device, and thus theposition of these reference points 17, 18 and 19 is measured. Thus, foreach measurement of the position of a point of a reference element 2 or3, the corresponding position and orientation of the reference object 1will be determined.

This makes it possible for the above-mentioned person, holding thereference object 1, to hold the latter in another position, for exampleafter the position of the first reference element 2 has been measured,in order to determine the position of the second reference element 3.

The distance between the two reference elements 2 and 3 is thencalculated by the measuring device, whereby the corresponding measuredrelative positions and orientations of the reference object 1 are takeninto account.

Next, as described above, the thus calculated distance between thereference elements 2 and 3 is compared to the actual distance, and theabove-mentioned mathematical model is adjusted.

Although the embodiment of the reference object, represented in FIG. 4,has three reference points, it is also possible to provide more thanthree reference points. It is also possible for the reference object 1to have only one or two reference points when the position of the bar 4is moved over short distances during the measurement, for example in theorder of magnitude of 1 mm.

It is clear that the mutual position of the reference points 17, 18 and19 on the reference object is not relevant, since only the relativeposition of each reference point is measured. The only thing required isthat their mutual distance is sufficiently large so as to allow for anaccurate determination of the position of the reference object 1.

During the measurements carried out with the measuring device accordingto the method of the invention, the reference points 12, 13, 14, 17, 18and 19 are preferably measured in turns.

When, during these measurements, the reference points are measuredsimultaneously, the different reference points are for example discernedby means of time-division multiplexing. This technique is known as such.

Although the description above is related to an optical measuringsystem, the method and the reference object can be applied in all sortsof measuring devices to determine the spatial position of the points ofan object. In particular, the method and the reference object accordingto the invention are not only suitable to be applied in opticalmeasuring systems, but for example also in co-ordinate measuringmachines.

The method and the reference object according to the invention areparticularly interesting to be applied in optical measuring deviceswhereby use is made of for example two or several flat cameras or threeor more linear cameras, or a combination of both of these.

Naturally, the invention is not restricted to the above-describedembodiments of the reference object and of the method according to theinvention represented in the accompanying drawings. Thus, the referenceobject may for example simply consist of a bar having a known length,and the reference elements consist of the far ends of this bar.

The above-mentioned reference points cannot only be formed oflight-emitting diodes (LED's), such as for example infrared LED's, butthey can also be formed of reflectors or coloured markings.

Further, the reference object can be equipped with more than tworeference elements, and they can have all sorts of shapes whatsoever,such as for example the shape of a cube, a tetrahedron, a cone, ahemisphere, etc.

1. A method for the verification and identification of an opticalmeasuring device for measuring, by means of one or several cameras, thespatial position of one or several points situated in the measuringvolume (9) of the measuring device, whereby the measuring device iscalibrated, according to which calibration a mathematical model iscalculated which makes it possible to determine the spatial position ofa point perceived by the measuring device, comprising, providing, duringa verification of the measuring device, a reference object (1) withpredetermined dimensions and with position in the measuring volume (9),determining the mutual position of the reference elements (2,3) bymeasuring the position of one or several points on the referenceelements (2,3) by means of the measuring device, the step of determiningcomprising placing a holder (11), upon which are provided referencepoints (12,13,14), against said one or several points on said referenceelements (2,3) and measuring the position of these reference points(12,13,14) in order to determine the position of the holder (11), andcalculating the mutual situation of the thus determined position of thereference points of the reference object (1) based on the measuredpoints comparing the calculated mutual situation of the thus determinedposition of the reference points of the reference object (1) to theactual dimensions of the reference object (1), wherein reference points(17,18,19) are provided on said reference object (1) and the position ofthese reference points (17,18,19) is observed by said cameras of themeasuring device to measure the position of these points, such that theorientation and the position of the reference object (1) is determinedfor each measurement, by means of said holder (11), of the position ofsaid points of the reference elements (2,3), and wherein the relativemovement of the reference points (17,18,19) of the reference object (1)and thus of the reference object (1) is determined, and wherein duringan identification of the measuring device, on basis of the thusestablished deviations between the measured mutual position of thepoints of the reference object (1) and the actual dimensions of thereference object (1), said model, that has been determined during thecalibration of the measuring device, is adjusted.
 2. The methodaccording to claim 1, wherein the position of said points of thereference elements (2,3) is determined for different positions and/ororientations of the reference object (1).
 3. The method according toclaim 1, wherein use is made of an optical measuring device having twoor more flat cameras or three or more linear cameras, or a combinationof both of these.
 4. The method according to claim 1, wherein theabove-mentioned mathematical model is only adjusted for those parametersof the measuring device which make the largest contribution to saiddeviations, whereby these parameters are for example the mutual anglesof the cameras in an optical measuring device.
 5. The method of claim 1,wherein said reference elements (2,3) are connected to one another bymeans of a connecting piece (4).
 6. The method according to claim 5,wherein said connecting piece consists of a bar (4).
 7. The methodaccording to claim 5, wherein said reference elements (2,3) are at leastpartially spherical, cube-shaped or conical or consist of a bore hole.8. The method according to claim 5, wherein said connecting piece (4)mainly consists of carbon fibres having a negative thermal expansioncoefficient, and of carbon fibres having a positive thermal expansioncoefficient, in such a ratio that its total expansion coefficient isnegligible, whereby these fibres preferably extend practically mainlyparallel to an imaginary connecting line between said referenceelements.
 9. The method according to claim 5 wherein each of saidreference points (17,18,19) is formed of an LED, in particular of alight source.
 10. The method according to claim 5, wherein saidconnecting piece (4) has a thermal expansion coefficient which isnegligible.
 11. The method according to claim 6, wherein said referenceelements (2,3) are at least partially spherical, cube-shaped or conicalor consist of a bore hole.
 12. The method according to claim 11, whereinsaid connecting piece (4) mainly consists of carbon fibres having anegative thermal expansion coefficient, and of carbon fibres having apositive thermal expansion coefficient, in such a ratio that its totalexpansion coefficient is negligible, whereby these fibres preferablyextend practically mainly parallel to an imaginary connecting linebetween said reference elements.
 13. The method according to claim 6,wherein said connecting piece (4) mainly consists of carbon fibreshaving a negative thermal expansion coefficient, and of carbon fibreshaving a positive thermal expansion coefficient, in such a ratio thatits total expansion coefficient is negligible, whereby these fibrespreferably extend practically mainly parallel to an imaginary connectingline between said reference elements.
 14. The method according to claim1, wherein use is made of an optical measuring device having two or moreflat cameras or three or more linear cameras, or a combination of bothof these.